Flows comprising particulate solids entrained in fluids such as gases are employed in various industrial and utility processes. An important example is the transport of pulverized coal from a pulverizer to the burner of a utility boiler. Often one pulverizer supplies several burners and flow adjustments are made by visually inspecting the flames issuing into the furnace. Another example is the return of hot partially gassified char from a cyclone separator that follows a coal gassifier back to the bottom of the gassifier.
In both of the aforementioned examples, it is highly desirable to obtain an accurate measurement of the flow rate of the entrained solids. In the case of the pulverizer-coal furnace, the measurement of pulverized coal flow would permit precise and continuous control of the air-to-fuel ratio at each burner. In the case of the gassifier, it would ensure that the separator is functioning properly as well as providing improved process data.
To date, the Applicants are not aware of any practical means for making such measurements. Systems are known such as are disclosed in U.S. Pat. No. 3,408,866, issued to Gibson et al which teach the use of a flow meter for gas-solid suspensions by measuring two pressure drop points in a venturi. However, to obtain mass flow, these known systems utilize a target indicator to measure such solid mass flow. As may be inferred from such prior art systems, the problem is that in a given place both the density and velocity of the flowing mixture are unknown. Furthermore, attempts to measure both of these variables encounter a certain amount of slip between the solid particles and the gases. This phenomenon is documented in articles by Carlson et al, entitled: "METER FOR FLOWING MIXTURES OF AIR AND PULVERIZED COAL", ASME Transactions, February 1948, pages 65-79. This article indicates that entrained particles do not follow the gas acceleration through an orifice so that the pressure drop reading indicates a gas flow rate. However, the particles are supposed not to slip relative to the gas in flowing through the venturi so that its indication gives the density of the flowing mixture once the volumetric flow is obtained from the orifice reading. However, in reality, solid particles actually do accelerate somewhat through the orifice and they do slip in traversing the venturi. Consequently, the meter described therein gives inaccurate readings and is sensitive to particle size.